[mythtv-users] ac3 passthrough problems

Scott list-mythtv at bluecamel.eml.cc
Thu Sep 28 21:27:19 UTC 2006

On Sep 28, 2006, at 11:07 AM, Brian J. Murrell wrote:
> On Tue, 2006-26-09 at 09:59 -0400, Scott wrote:
>> 1) In MythTV locate a 5.1 source that fails to play correctly
> Well, how about just using a DVD with an AC sound track?

This will work well.

>> 3) Using mplayer, run "mplayer -v -ao alsa -afm hwac3
>> source_file_from_mythtv.mpg" and verify the sound works as expected
> I did this:
> # mplayer -v -aid 128 -channels 6 -vo null -ao alsa -afm hwac3 -dvd- 
> device /dev/cdrom dvd://1

The -aid 128 should be sane but I'm not sure about -channels 6. That  
maybe confusing mplayer a bit. Really,
all you should need is "mplayer -v -ao alsa -afm hwac3 -dvd-device  
dvd://1" unless it's an odd disc. (Caution, I define odd as anything  
that isn't mainstream region one!)
> Trying to force audio codec driver family hwac3...
> Opening audio decoder: [hwac3] AC3/DTS pass-through S/PDIF
> dec_audio: Allocating 8192 bytes for input buffer.
> dec_audio: Allocating 16384 + 65536 = 81920 bytes for output buffer.
> No accelerated IMDCT transform found
> hwac3: switched to AC3, 448000 bps, 48000 Hz
> AUDIO: 48000 Hz, 2 ch, ac3, 448.0 kbit/29.17% (ratio: 56000->192000)
> Selected audio codec: [hwac3] afm: hwac3 (AC3 through S/PDIF)

Good. I expect you heard sound if it did this.

> Building audio filter chain for 48000Hz/2ch/ac3 -> 0Hz/0ch/??...
> [libaf] Adding filter dummy
> [dummy] Was reinitialized: 48000Hz/2ch/ac3
> [dummy] Was reinitialized: 48000Hz/2ch/ac3
> alsa-init: requested format: 48000 Hz, 2 channels, 100
> alsa-init: using ALSA 1.0.11
> alsa-spdif-init: playing AC3, 2 channels
> alsa-init: using device iec958:{CARD 0 AES0 0x02 AES1 0x82 AES2  
> 0x00 AES3 0x02}
> alsa-init: pcm opend in blocking mode
> alsa-init: chunksize set to 1024
> alsa-init: fragcount=16
> alsa-init: got buffersize=65536
> alsa-init: got period size 1024
> alsa: 48000 Hz/2 channels/4 bpf/65536 bytes buffer/Signed 16 bit  
> Little Endian
> AO: [alsa] 48000Hz 2ch ac3 (1 bytes per sample)
> AO: Description: ALSA-0.9.x-1.x audio output
> AO: Author: Alex Beregszaszi, Zsolt Barat <joy at streamminister.de>
> AO: Comment: under developement
> Building audio filter chain for 48000Hz/2ch/ac3 -> 48000Hz/2ch/ac3...
> [dummy] Was reinitialized: 48000Hz/2ch/ac3
> [dummy] Was reinitialized: 48000Hz/2ch/ac3
> Starting playback...

This looks exactly like what my mplayer output gives me. Since my  
config works with mplayer I would say that's good! ;)

>> At step 3, if the sound doesn't work as expected then most likely the
>> problem is ALSA is not configured correctly yet.
> But alsa is definitely passing digital PCM to my receiver.  Would that
> not mean that everything is ready to go to pass AC3 as well?

I agree, it should mean that the physical hookups and the routing of  
audio is working as expected. Clearly though, if you cannot get  
either MythTV or mplayer to spit out AC3 DD 5.1 to your receiver than  
there is a deeper issue than just MythTV.

After looking at your asoundrc and Alsa devices I would suggest, for  
now, ditching the asoundrc file. When testing with mplayer and a  
hardware passthrough it shouldn't be needed. Often, and as your log  
shows, mplayer is smart enough to find the spdif device and send the  
audio out. Other than that, try dropping some of the options from  
mplayer command line and see if your receiver detects DD then.


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